Abstract

Due to the peculiar shape of the phonon-roton dispersion curve in 4He, the spontaneous decay of one excitation into a number of others is forbidden for wavenumbers above qc ≃ 0.55 Å−1. Therefore, q > qc excitations can only decay by scattering on another, thermal excitation, and consequently, in the ground state T→ 0 their lifetime becomes infinite, as put to evidence by ballistic photon/roton experimentsl. In contrast the q ≤ qc phonons must have a finite lifetime at T→ 0 and we shall present the first quantitative observation of this phenomenon.